Climate

The climate of the Mekong River Basin ranges from temperate to tropical. In the Upper Mekong Basin, some of the taller peaks of the Tibetan Plateau are glaciated. In fact, much of this part of the Basin is snow-covered in winter. Melting snow from the Tibetan Plateau feeds the Mekong River’s dry-season flow, especially in the middle reaches. In the relatively lower elevations of the Yunnan province of China, the climate of the Mekong River Basin changes and the temperature gradually increases.

June to October is the wet season in the Lower Mekong Basin (LMB); with the exception of two brief transition periods (see Figure 1 below), the rest of the year in the LMB is the dry season. The wet season results from the flow of moisture-laden air from the Indian Ocean in the summer. During the rest of the year, high-pressure systems over the Asian continent give rise to the dry season in the LMB.

Figure 1. Lower Mekong Basin Seasonal Climate. Source: MRC, 2010.

The climate of the Lower Mekong Basin (LMB), which is almost always hot and often humid, is classified as tropical monsoonal. In the warmest months of March and April, average temperature ranges from 30°C to 38°C. Cooler temperatures prevail from November to February. At higher elevations in the Lao People’s Democratic Republic (PDR), winter temperature averages 15°C.

The distribution of mean annual rainfall over the Basin follows a distinct east-to-west gradient. In the LMB, the rain-soaked uplands in the Lao PDR and Cambodia receive the most precipitation (3,000 mm) and the semi-arid Khorat Plateau in northeast Thailand the least (1,000 to 1,600 mm). The Upper Mekong Basin is similar to the LMB in that rainfall is regulated by the global monsoon system. In the Upper Basin, annual rainfall can be as little as 600 mm in the Tibetan Plateau and as much as 1,700 mm in the mountains of Yunnan.

Climate change in the Mekong River Basin

Lower Mekong Basin mean annual rainfall. Source MRC 2010.

Countries in the LMB are among the most vulnerable locations in the world with respect to climate change. While the future impact of climate change is difficult to forecast, projections for the Mekong River Basin for the next 20 to 30 years, based on a downgraded global climate model, are as follows:

Basin-wide temperature increase of 0.79°C, with greater increases in colder catchment areas in the north

Annual precipitation increase of 200 mm (a 13.5% rise)

Increase in dry-season precipitation in northern catchments and decrease in southern catchments

Total annual runoff increase of 21%

Increase in flooding in all parts of the Basin with the greatest impact on downstream catchments of the Mekong River

Climate change is expected to affect natural ecosystems and agriculture throughout the Mekong River Basin, thereby exacerbating the challenges of meeting the increasing demand for resources from growing populations (Hoanh et al. 2003).

More than 120 participants from various stakeholder groups gathered today in Luang Prabang to further debate the proposed Pak Lay hydropower project as the Mekong River Commission (MRC) convened the 2nd regional stakeholder session on the project, as part